A computer system includes a number of components, including a central processing unit (CPU), memory, and peripheral devices. Examples of peripheral devices include storage media cards, video cards, modem cards, network cards, and other adapter cards. In high-performance computer systems, a large number of adapter cards can be installed in the system. In most cases, adapter cards are mounted to adapter card brackets, which in turn are attached to the main chassis of the computer system. To receive the adapter card brackets, openings are provided in the chassis. Such openings are typically provided in what is referred to as a “card cage portion” of the chassis. The card cage portion has multiple openings to receive multiple adapter cards. The adapter card brackets are attached to corresponding structures of the card cage portion.
One of the issues associated with the openings in the chassis is the likelihood of electromagnetic emissions. A primary concern of electromagnetic emissions is the potential for electromagnetic interference (EMI). To reduce radiated emissions, intimate electrical contact must be achieved between the adapter card bracket and the main chassis. To achieve such intimate electrical contact, some manufacturers of computer systems provide factory-installed radiation gaskets, particularly on servers and nodes designed for multiple CPUs and/or multiple adapter cards. A radiation gasket is formed of an electrically conductive material and is placed between the adapter card bracket and the chassis to enhance the electrical contact between the adapter card bracket and the chassis.
As computer technology has evolved, the speeds at which components in computer systems can operate has increased dramatically. Frequencies at which individual components can operate as well as the frequency of busses (which provide the interconnection between components of a computer system) can be relatively high. The higher the frequency, the greater the amount of noise generated by operation of the computer system. The high-frequency noise resulting from such operation results in increased electromagnetic emissions.
As computer system operating frequencies have increased, factory-installed gaskets often do not provide adequate protection against electromagnetic emissions from the chassis openings. If a computer system has a dense arrangement of adapter cards, an even greater amount of radiation leakage may result. In addition, computer systems must exhibit immunity from external electromagnetic emissions, such as an electrostatic discharge (ESD) near the adapter card cage. In many instances, the factory-installed gaskets do not provide adequate protection against ESD.